1. Field of the Invention
This invention relates to the general field of nondestructive radiation inspection using x-ray or .gamma.-ray imaging systems,
2. Description of Related Art
A single x-ray (or .gamma.-ray) energy band with finite band width has been used in conventional nondestructive projection imaging. Such imaging systems use the accumulated attenuation information of all the materials within the region of imaging interest and cannot distinguish one material from another when two different materials contribute to the same amount of attenuation. For example, a thin metal plate and thick plastic plate may not be distinguishable with single energy projection imaging.
An improvement from single energy projection imaging is the use of two different x-ray energy bands (a high energy band and a low energy band) and the ratio of material attenuation coefficients at these different energies, .mu..sub.H /.mu..sub.L (U.S. Pat. No. 4,982,584). When the ratio is close to one, the object is identified as plastic; when the ratio is less than one, the object is identified as metal. Such a method can discern plastic from metal only if the region of interest does not contain overlapping of both materials. Also, when the high energy band is above and near an absorption edge of the metal (e.g. a K edge), the ratio of the material attenuation coefficients may be close to one (indicating plastic), even though the region of interest contains metal. The distinction between plastics and metals is thus made difficult when the high energy band used is close to a K edge of a metal in the region of interest.
A further improvement along the attenuation coefficient ratio method can be found in U.S. Pat. No. 5,319,547. This technique depends on the comparison of x-ray transmission between a reference region and its neighbor at different x-ray energies. This method cannot solve for the thickness information of materials or distinguish among more than two different materials existing in the same region of interest.
In the medical field of angiography, two appropriately filtered different x-ray energies have been used to identify iodine contrast in a region of interest containing iodine, bone, and tissue. This image subtraction technique incorporates the difference of linear absorption coefficients at different x-ray energies. Two different x-ray energies at which the bone and tissue attenuation coefficients are very close to each other are used to produce two different x-ray images. A weighted subtraction of the images is performed to eliminate the bone and tissue images and to sort out the iodine image. When residual bone background images remain after subtraction, a third energy below the iodine absorption K edge may be added to cancel the residual bone background and to isolate the iodine-filled blood vessel image (U.S. Pat. No. 4,686,695, 1987). This technique is specified with materials existing in the human body that are known prior to imaging such as bone, tissue, iodine contrast, and lesion. According to this particular technique, the two x-ray energies should be chosen such that the bone and tissue mass attenuation coefficients are close to each other at the two x-ray energies. Another limitation is that when the third energy is needed for eliminating residual bone images, it should be chosen to be below the iodine or lesion absorption edge. Moreover, this technique does not operate and produce results in real-time. None of the prior art mentioned above provides a method or device capable of performing real-time image separation for an unknown ensemble of materials that contains a large number of different materials.